The embodiments described herein relate generally to electric power generation and delivery systems and, more particularly, to systems and methods for using distributed energy resources (DER) in an electric network that includes variable generating systems.
Power generated by an electric generating entity is typically delivered to a customer via an electric network or grid that consists of transmission and distribution circuits. The electric power generation and transmission system is closely monitored and controlled by an electric grid control system that includes a large number of individual subsystems, which may also include multiple components. Typically, information is transmitted from many of the subsystems/components to the control system for use in controlling operation of the electric grid. For example, some electric generation entities utilize an Energy Management System or Control Center.
Known Energy Management Systems include a plurality of components and subsystems that communicate with, and may be controlled by, a central management system, typically located at the electric generating entity. The components and subsystems may be distributed at various points in the electric network to facilitate power transmission. Due at least in part to the large scale of an Energy Management System, and the quantity of individual component/subsystems that may be included, information at the management system, for use in centralized management of the generation and transmission, is generally expansive and complex.
Traditionally, for distribution systems, voltage and reactive power (Volt-VAr) control have been performed to overcome both over-voltage and under-voltage violations through controllable reactive power sources present in the system. By controlling the production, absorption, and flow of reactive power present in the system, Volt-VAr control can maintain the voltage profile within acceptable limits and reduce the distribution system losses. Traditional Volt-VAr control is achieved by reconfiguring controllable devices such as voltage regulators and Load Tap Changers of transformers (LTC) for voltage control, and shunt reactors and shunt capacitors for VAr control.
However, feeder voltage and reactive power flow are closely related and dependent variables for which control actions to change one of the variables can result in opposing control actions to change the other variable. For example, raising the voltage using the substation transformer LTC can produce a voltage rise that could cause capacitor bank controls to remove a capacitor bank from service, thus lowering the voltage. Similarly, placing a capacitor bank in service could cause the LTC to lower the voltage at the substation.
While such conflicting control actions generally do not produce unacceptable electrical conditions on the feeder, they do produce conditions that are less efficient. The coordinated control of voltage and reactive power is needed to determine and execute volt-VAR control actions that are truly optimal.
Furthermore, known distribution management systems (DMS) based VVC solutions are not very scalable and have high implementation and operation costs that hinder electric generating entity adoption. Conventional local volt-var control techniques are not capable of voltage flattening, CVR, reactive power reduction and unity power factor that increases the efficiency of the system.
Generally, a majority of customers (i.e., loads) are located at the distribution circuits. Power utilities desire to monitor and control the components that are distributed along the distribution circuits. For this purpose, some power utilities utilize what is referred to as a “smart grid.” At least some known smart grids include a plurality of components and subsystems that communicate with, and may be controlled by, a central management system, typically located at the electric generating entity. The components and subsystems may be distributed at various points in the electric generating entity distribution network to facilitate power distribution to customers. Due at least in part to the large scale of a smart grid, and the quantity of individual component/subsystems that may be included in the smart grid, information at the management system, for use in centralized management of the smart grid, is generally expansive and complex.
Electric power losses across distribution feeders in an electric network, is a concern for distribution systems engineers. Between about three percent and about eight percent of power transmitted on distribution feeders is lost. The electric power losses include ohmic losses, losses from reactive power flow, and losses due to harmonic currents resulting from nonlinear loads of the system. Presently, various voltage/Var control schemes are sometimes used to reduce transmission losses. In at least one known scheme, Var compensation is implemented by the use of the capacitor banks that are placed on critical buses of an electric network system to supply reactive power to support and attempt to optimize the voltage profile of the system. Real time control actions can be implemented, to some extent, through switched capacitor banks. However, such capacitor banks, including switched capacitor banks, are placed only at discrete points of the electric network and inject discrete levels of reactive power. Moreover, the control of switched capacitor banks is commonly based on information local to the particular switched capacitor bank.
With the addition of fast dynamics distributed energy resources (DER) additional control is needed to account for estimated control inputs from the DERs. For example, many slow dynamics electromechanical devices are capable of controlling voltage on the grid over relatively long periods of time. Such legacy type devices are able to account for daily load variations that are generally well characterized, such as load variations due to heavy electrical load increases as factories come on line in the morning and load decreases due to factories and other large loads securing in the evening. However, the power, reactive power, and voltage support capabilities of DERs can vary over very short intervals of time. A photovoltaic installation may be affected by clouds passing over the collecting field or a wind farm may be affected by variable winds.